Compact discs are record carriers for digital or analog information, for example audio and/or video information, which have the form of a flat disc-shaped plastic surface on which a data modulated optical structure is provided in accordance with the information. The data is formed as pits of varying length present in the surface and arranged in a spiral path. The pits have nanometer dimensions and are formed by injection molding against a mold having corresponding raised regions. The information stored on the compact disc is recovered in a reader, usually called a player, which rotates the compact disc and guides a laser device along the spiral track as the compact disc rotates. The presence or absence of pits under the laser is detected as a change in the luminance returned from the surface directly below the laser. In this manner the length of the pits is detected and decoded as data.
A newly developed data format for compact discs makes use of a very thin data layer into which the pits and lands are formed during injection molding. These pits have dimensions on the order of 250 nm, and the thickness of the data layer and its metallization coating is on the order of 0.6 mm. This thickness is advantageous to reduce the effect of coma of the reading laser beam if the disc becomes tilted during reading or deviates from planarity. In order to provide sufficient stiffness to the disc, however, it is necessary that it be thicker. The additional thickness is provided by adhesively bonding another disc surface to the data containing surface. The bonding operation however introduces tilt (i.e. deviation from planarity) to the data recorded layer of the disc. The present invention is concerned with the reduction of such tilt in the manufacture of compact discs made from thin disc layers bound together by a light curable adhesive.
During the manufacture of certain high density optical discs two surfaces on at least one of which the data has been recorded are bonded together using a light sensitive adhesive. The adhesive, which is sandwiched between the two layers of the disc is cured by heat and/or ultraviolet light. During the bonding process heat is generated that temporarily distorts the two disc layers in an asymmetrical fashion. For example, the inner diameter of the disc expands differently (generally with a greater vertical displacement) than the outer diameter of the disc. If curing is achieved during this "asymmetrical" time period, the composite disc realizes a significant distortion.